This invention relates to a unidirectionally oriented carbon fiber-reinforced carbon composite material and to a method of producing same.
Carbon fiber-reinforced carbon composites hereinafter referred to as C/C composites) in which reinforcing carbon fibers are unidirectionally oriented and dispersed within a matrix of carbon have excellent mechanical properties, high resistance to heat, thermal shocks and abrasion and high heat conductivity and, thus, are used in a wide variety of applications, for example, heat sinks, aerospace parts, brakes of aircraft and racing cars and fusion reactor parts.
One known method for the production of such C/C composites is a so-called CVD method in which a premold of carbon fibers having a desired shape is heated in a furnace to a high temperature while feeding a hydrocarbon gas to the furnace, so that the hydrocarbon is thermally cracked to form carbon which deposits on the surface of the fibers of the premold. The CVD method, however, has a problem because it is difficult to uniformly deposit carbon in the depth of the fiber premold so that defects such as cracks and voids are apt to be formed. As a consequence, the resulting C/C composite fails to show satisfactory mechanical strengths in the direction perpendicular to the orientation direction of the fibers. Moreover, the CVD method requires large costs and is time consuming and, thus, is not economically advantageous.
Another method is known for the production of C/C composites, in which a thermosetting resin is used as a precursor for the carbon matrix. Since the carbon yield of the thermosetting resin is as low as about 50%, however, the C/C composites thus obtained contain cracks and voids. To eliminate such cracks and voids, it is necessary to repeat the impregnation and calcination for a number of times, generally 5-10 times. This is of course time consuming (several months are required) and is economically disadvantageous. Further, the mechanical strengths of the resulting composites are not fully satisfactory.
C/C composites may also be produced by a method in which carbon fibers are impregnated with a molten carbonaceous pitch serving as a matrix precursor, the resulting impregnated fibers being subsequently calcined. Since the carbonaceous pitch when melted should exhibit flowability suitable for impregnation, the carbon yield is low. Thus, cracks and voids are unavoidably formed in the C/C composites.
To cope with the foregoing problems, a method is proposed in which a dispersion of raw coke powder in a solution of a thermosetting resin, such as a phenol resin, dissolved in furfural or furfuryl alcohol is impregnated into carbon fibers, the resulting impregnated fibers being subsequently calcined (JP-A-3-247563). The average particle diameter of the coke powder is 30 .mu.m or less, preferably 0.5-15 .mu.m. It is disclosed in JP-A-3-247563 that the use of carbon powder having an excessively small particle size causes a difficulty in obtaining a dense composite material. According to this method, since the coke powder gives a high carbon yield, a C/C composite with an improved quality may be obtained. With this method, however, it is impossible to prepare C/C composites having a dense matrix especially when the fiber content is increased to improve mechanical properties and thermal conductivity.
Thus, it is necessary to repeat the impregnation and calcination in order to improve the mechanical properties. When the impregnation and calcination are repeated, large pores of an open cellular structure can be filled with new layers of the carbon matrix. However, this results in the formation of closed pores. When a C/C composite having closed pores is used under vacuum, such as in a fusion reactor or in space, the gas contained therein expands to cause the formation of cracks or the deterioration of mechanical strengths. Further, the repeated impregnation and calcination will cause a difference in physical properties, such as coefficient of thermal expansion, between the newly formed carbon layers and those of the previously formed matrix, so that a residual internal stress is accumulated in the C/C composite.
Incidentally, in high precision machining, such as precision drilling, which requires a high processing accuracy in the order of micron units, it is essential that the average pore diameter of the material should be as small as possible and should not be greater than the required processing accuracy. The conventional methods cannot produce C/C composites having such a small average pore diameter.